The present invention relates generally to improvements in subscription television systems and, more particularly, to improvements in apparatus for unscrambling a broadcast RF television signal having suppressed horizontal blanking and synchronization pulses.
In subscription television systems television programming signals are transmitted, either "over-the-air" or through a suitable cable network, in a scrambled form, which signals may be unscrambled for viewing by a suitable decoder operating the television receiver of an authorized system subscriber. The programming signals may be grouped into several levels or tiers each representing a different programming category such as sporting events, movies, etc., with the decoder of a particular subscriber being authorized for unscrambling the televised programming signals in selected categories, the televised programming signals in the remaining unauthorized categories being coupled to the television receiver in a scrambled and unviewable form. Of course, in such a system, all the programming signals will be reproduced in a scrambled and unviewable form in a normal television receiver.
Horizontal synchronization suppression techniques are commonly used to effect a scrambled broadcast television signal by suppressing the horizontal blanking and synchronization pulses below the average video level of the television signal. This causes the horizontal deflection system of a television receiver to lock on random video peaks during the active or video trace line portion of the video signal rather than on the actual horizontal synchronization pulses. In addition, the ability of the television receiver to use the color reference burst associated with the horizontal synchronization signals is severely degraded thereby causing inaccurate color reproduction. U.S. Pat. No. 4,222,068, to Thompson, teaches a system employing the foregoing techniques wherein an encoder is provided for modifying a broadcast television signal by suppressing the amplitude of randomly selected horizontal blanking and synchronization pulses during the fields of the video signal. Each system subscriber is provided with a decoder whose front end comprises conventional tuning, intermediate frequency and video detection stages. Also, a gated automatic gain control circuit, adapted for responding to the tips of standard NTSC horizontal synchronization pulses, is provided for controlling the gain of the tuning and intermediate frequency stages in accordance with the output of the video detection stage. A sync restoration circuit is operable in response to the suppressed front porch of each suppressed horizontal blanking pulse developed at the output of the video detection stage for producing a standard NTSC horizontal synchronization pulse, which standard horizontal synchronization pulses are continuously coupled to an input of the automatic gain control circuit for insuring proper operation thereof and also continuously inserted in the video signal developed at the output of the video detection stage for achieving an unscrambled video signal. The video signal including the restored horizontal synchronization pulses is impressed on a standard television channel RF carrier by a remodulator and then coupled as an unscrambled RF television signal to the antenna inputs of a television receiver. Each decoder additionally includes a decode authorization circuit storing a unique subscriber code which is compared to a subscriber authorization code transmitted during a horizontal line of the vertical interval of the broadcast television signal. If the stored subscriber code and the transmitted subscriber authorization code results in a favorable comparison, a decode authorization signal is developed enabling the remodulator for coupling the unscrambled signal to the television receiver for viewing, the remodulator otherwise being disabled. The system disclosed in the Thompson patent therefore requires the use of a relay or other relatively expensive switching circuit for selectively enabling and disabling the remodulator. In addition, when the remodulator is disabled in response to the absence of a decode authorization signal, a noisy video image is nevertheless produced on the viewing screen of the television receiver which frequently leads to the annoying situation wherein the viewer mistakenly assumes that the decoder is malfunctioning.
In order to alleviate these and other problems, modifications of the decoder taught in the Thompson patent have been proposed wherein the restored horizontal synchronization pulses are gated for operating the automatic gain control circuit and for insertion in the video signal path for unscrambling the broadcast television signal only in response to the development of a decode authorization signal. These modified decoders also typically include a video switch connected in the video signal path between the video detection stage and the remodulator for selectively controlling the application of the video signal to the input of the remodulator. Thus, when the video switch is operated for establishing the video signal path a scrambled video signal is coupled to the remodulator in the absence of a decode authorization signal and an unscrambled video signal is coupled to the remodulator by gating the restored horizontal synchronization pulses for operating the automatic gain control circuit and for insertion in the video signal path in response to the development of a decode authorization signal. In accordance with this decoding technique, it will be appreciated that during the absence of a decode authorization signal the automatic gain control circuit will not function properly due to the suppressed level of the horizontal synchronization pulses. Moreover, it has been found that upon the development of a decode authorization signal, an extended delay (sometimes of up to several minutes) is encountered before the automatic gain control circuit initiates proper operation in response to the restored horizontal synchronization pulses. As a result, an unacceptable picture is produced on the viewing screen of the television receiver for a time interval even though the subscriber has been authorized to decode the broadcast signal. This condition is quite annoying to the viewer and hence poses an undesirable situation.
It is therefore, a basic object of the present invention to provide an improved decoding apparatus operable for unscrambling a broadcast RF television signal characterized by suppressed horizontal blanking and synchronization pulses.
It is a more specific object to the invention to provide a decoder of the foregoing type wherein the remodulator associated therewith is continuously operable independent of the development of a decode authorization signal and wherein the automatic gain control circuit associated with the tuning stage of the decoder is responsive for immediately assuming a proper operating mode upon the development of a decode authorization signal.